Best Carb for Modded 306 cid - Vintage Mustang Forums
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post #1 of 53 (permalink) Old 06-25-2019, 05:03 PM Thread Starter
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Best Carb for Modded 306 cid

Recently completed my engine and tranny build and am looking for some recommendations on Carbs. I'm thinking that a 650 Double Pumper Mechanical would do the trick, but want to make sure I'm not missing anything.

Engine Specs
306 cid Roller
AFR 165 Heads
Lunati 218/218 Cam (1800-5800 with 1.6 rockers) 351w firing order
1.7 Rockers
Edelbrock Air Gap Intake
Ford Powertrain Applications 1 5/8" Headers

Tranny
Stage II C4 with 2400 stall and GV OD

Last edited by Shinobi68; 06-25-2019 at 05:06 PM.
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post #2 of 53 (permalink) Old 06-25-2019, 05:21 PM
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This would be my choice https://www.holley.com/products/fuel...s/parts/BD-650

Last edited by cmefly; 06-25-2019 at 05:25 PM.
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post #3 of 53 (permalink) Old 06-25-2019, 05:27 PM
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What gear ratio? Iíd go with a 600 cfm, annular boosters with vacuum secondary.
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post #4 of 53 (permalink) Old 06-25-2019, 05:31 PM
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I have bought 4 reman carbs from Holly on ebay with no issues. All looked New. They have some good prices.
https://www.ebay.com/itm/Quick-Fuel-...QAAOSwpqddArM-
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post #5 of 53 (permalink) Old 06-25-2019, 05:50 PM
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I have bought 4 reman carbs from Holly on ebay with no issues. All looked New. They have some good prices.
https://www.ebay.com/itm/Quick-Fuel-...QAAOSwpqddArM-
I was going to comment that they were returned simply because the original installer didnít have a clue on carburetors and simply had the idle mixture set wrong which is pretty much what was said in the description.

I donít know how many times on FB someone has a carb issue and they have no clue about setting idle mixture, setting timing or even know what a timing light is let alone explaining about points. Gawd Iím getting old
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post #6 of 53 (permalink) Old 06-25-2019, 06:06 PM Thread Starter
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Thanks will check it out.

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What gear ratio? Iíd go with a 600 cfm, annular boosters with vacuum secondary.
3.55:1, curious why 600 over a 650? I have a 570 Holley but am concerned it is too small.

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I have bought 4 reman carbs from Holly on ebay with no issues. All looked New. They have some good prices.
https://www.ebay.com/itm/Quick-Fuel-...QAAOSwpqddArM-
Will check these out as well. Thanks

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I was going to comment that they were returned simply because the original installer didnít have a clue on carburetors and simply had the idle mixture set wrong which is pretty much what was said in the description.

I donít know how many times on FB someone has a carb issue and they have no clue about setting idle mixture, setting timing or even know what a timing light is let alone explaining about points. Gawd Iím getting old
Saw that and was going to look for a bargain on returned items.

Just curious now why the 600 recommendation over a 650? Appreciate you insight on this.
Thanks

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post #7 of 53 (permalink) Old 06-25-2019, 06:34 PM
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I'm guessing because people have been putting carburetors that are too large on their cars as long as I've been alive. Drag strip performance is good that way but it makes them so much less responsive (less fun) to drive. There are rule of thumb calculators out there where you can calculate what size your engine should require and I'd bet all of them will say even your 570 is on the large side.

It's personal preference, but I like a "sporty" feel from my cars. Quick throttle response at any RPM is a big part of that. I have ridden (and driven) cars built with their focus on drag racing that were dogs on the street. In all cases the owners were quite oblivious that their cars could run ever so much better on the street and had no interest in changing anything. To each their own, I've no interest in drag racing.

Old school rule of thumb is not to use a double pumper unless the car has a manual transmission or a very high stall converter.
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post #8 of 53 (permalink) Old 06-25-2019, 07:01 PM
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I would go QF 650 vacuum. Race version without choke if you want best top end and don't mind warming it up on cold days. That's what I use, also in N Cal. Does QF even make a carb smaller than 650? As to GypsyR, the cam would dictate bottom end throttle response more than carb type. Vacuum is always best for street, IMO!

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post #9 of 53 (permalink) Old 06-25-2019, 07:26 PM
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Quote:
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Does QF even make a carb smaller than 650?
Yes, I've run a QF Slayer 450 cfm on a rebuilt stock 289 until I found an Autolite 4100 480 cfm. The QF Slayer ran fine, however I did increase the jets one size and the accelerator cam 2 sizes. My AFR showed that the engine was pretty well dialed in with those 2 changes.
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post #10 of 53 (permalink) Old 06-25-2019, 07:35 PM
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You can use any online CFM calculator to find out how many CFM you need based on RPM, cubic inches, and approximate volumetric efficiency. It's unlikely that a street 302 (or 306 in your case) will need a carb over 600 CFM. In fact, few 302's really need more than about 550 peak CFM, even if they're pretty healthy.

Having a 'larger' carburetor means less pressure drop in the intake (less vacuum at wide open throttle), which can make more power - to a point. When airflow going through the carburetor isn't enough to draw fuel from the boosters properly, you end up with problems. The type of booster being used also affects this. A typical Holley booster doesn't atomize the liquid very well, when it has low airflow. Big fat droplets of gas don't burn as well as a fine mist (or vapor), so your low and midrange performance suffers. Going to a "big" carb may offer a few more horsepower on the top end, but at the expense of throttle response, economy, and average power.

Using mechanical secondaries has much the same effect. Instead of feeding the engine what it needs, you're just dumping a big dollop of gas down the throat as you snap the blades open, and saying "I don't care what you want, just do it!" That can be okay for a manual transmission, especially on the track where you're not going to be using anything except for idle and WOT, but it isn't so great for an automatic in traffic.

While peak horsepower is king on the drag strip (usually), average horsepower means a lot more to any street driven car. Going with a carb that matches your engine, especially one with annular boosters that deliver a very fine mist of fuel will really help wake up your car, and give you better gas mileage to boot. Most of the Holley-based carbs (like the Demon, and even the Holley Dominator) offer annular boosters on some of their bigger offerings, but the Summit M-series carbs are a lot less expensive, and very effective. They come in different sizes starting at 500 and 600 CFM. For a mild build, the 500 would be great, but rounding up (to ensure more air than you will likely need) you'd want to go with a 600 CFM version.
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post #11 of 53 (permalink) Old 06-25-2019, 08:07 PM
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Iím running a Holley 570 street Avenger on my similarly built 306.
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post #12 of 53 (permalink) Old 06-25-2019, 08:32 PM
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Quote:
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Using mechanical secondaries has much the same effect. Instead of feeding the engine what it needs, you're just dumping a big dollop of gas down the throat as you snap the blades open, and saying "I don't care what you want, just do it!"
I think thatís the best saying Iíve seen on this forum. I almost spit my coffee out......

I think the op would be ok with a 650 DP carb. Probably have a little better throttle response around town with something like one of those 600 carbs from summit and it would be more forgiving but Iíd wager the 650 would outrun it mid range and top end.
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post #13 of 53 (permalink) Old 06-25-2019, 09:01 PM
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My question for all the Holley fans, why not just use Holley's calculator? Seems to me that they should know what they're talking about.
https://www.holley.com/products/fuel...s/carburetors/
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post #14 of 53 (permalink) Old 06-25-2019, 09:22 PM
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My question for all the Holley fans, why not just use Holley's calculator? Seems to me that they should know what they're talking about.
https://www.holley.com/products/fuel...s/carburetors/

If all you want is a Holley, it's fantastic.

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post #15 of 53 (permalink) Old 06-25-2019, 11:06 PM
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I'm guessing because people have been putting carburetors that are too large on their cars as long as I've been alive.

Truer words have never been written!
I remember the days when everybody thought they had to have a "750 Double Pumper" on their 283/289 or they were likely to be called a wimp.
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